In 2012 the world aluminum oxide production reached and exceeded 70M metric tons. Aluminum oxide, which is also called alumina, is mainly used as a feedstock material for aluminum metal production. The main source of alumina is an ore called bauxite which contains roughly between 25-75% weight content of aluminium oxide. The ore cannot be used directly by smelters and must be enriched to reach a 99% weight content of aluminium oxide. It is known to increase the alumina content with the Bayer process. In typical applications, the bauxite is grinded, mixed with a solution containing between 200-300 g per liter of sodium hydroxide and heated under pressure at a temperature above 140° C. At these conditions of ore digestion, the alumina compounds are solubilised while typical impurities like iron oxides, titanium oxides and others, are not. All of these impurities will remain in the liquid in the form of solid suspension. After the digestion process, the solution containing alumina and impurities are cooled to atmospheric boiling point and then subjected to two solid-liquid separation steps. The addition of flocculent into a tank called “deep thickener” is the first separation step. Under the flocculent effect, the agglomerated particles settle at the bottom of the tank where they are pumped out. The top clear part of the solution is now almost free of solid impurities with only traces of them still in the solution. The overflow of clear solution is sent to the second step of filtration where the traces of impurities are removed. A concentration of impurities of less than 5 mg per liter can remain after filtration.
It was traditionally known to perform the latter filtration step, often referred to as security filtration, by leaf filters. In the last several years, many new alumina production plants are engineered with a different type of filter referred to as channel filters. Nonetheless, an impressive amount of plants still use leaf filters today.
An example leaf filter is shown in FIG. 1, where the leaf filter is shown during a cleaning step, with a cylindrical shell of the filter slid lengthwisely, exposing the internal structure of the leaf filter which houses a plurality of ‘leafs’ (typically about 15-25) of various sizes engineered to adapt to the space available in the cylindrical shell.
The leafs of the filter each includes a mesh frame having a steel structure supporting a steel mesh, such as shown in FIG. 2, which is covered by a filtration fabric provided in the form of a bag such as shown in FIG. 3 for use. During operation, the cylindrical shell is slid into position to close the vessel, and the solution intended to be filtered is pumped into it. When the vessel is full, the pressure rises due to a pumping action, and the solution flows through the fabric, across the mesh, and the filtrate is extracted through an outlet pipe. The purified solution, full of solubilised alumina, is ready for production of solid alumina. Filtration by the leaf springs leads to formation of a rock-solid accumulation known as scale on the bags. Accordingly, the bags are consumable items which are subject to regular replacement, typically at each 4-6 weeks. A typical leaf filter can require 250-500 square meters of fabric to cover all of the 15-25 leafs. A typical plant has at least 5 leaf filters, and can thus use about 1000 filter bags per year. Many plants have 10 to 15 filters and can thus use 2000 to 3000 filter bags per year.
Although the known leaf filters were satisfactory to a certain degree, they met certain limitations. For instance, in typical production plants, a significant amount of leafs were detected to have leaks after only a few hours of operation. These leafs were then closed for the remainder of the 4-6 weeks of operation of the leaf filters, leading to loss of production and waste of fabric. Moreover, the process of replacing the bags was labour-intensive, especially in a context where a significant amount of scale needed to be removed to allow extracting the steel frame from the used bags. Henceforth, there remained room for improvement.